Particulates emitted from industrial stacks are monitored for a number of reasons. Several countries impose regulatory restrictions on emissions, and monitoring is necessary to ensure compliance. Increasingly, companies have their own internal environmental programs and monitor emissions to provide data for those, and for their internal quality control procedures. Companies also monitor emissions in order to improve their own processes, for example by improving powder production, reducing product loss from process-particle collection devices and reducing running costs of fabric filters (by extending bag life).
Several different techniques have been used for monitoring emissions from industrial stacks, and more particularly for measuring particle flow in stacks. Two important techniques are those using electrodynamic instruments and those using forward-scatter monitors.
Electrodynamic instruments measure an ac current resulting from particles interacting with a probe rod that projects into the stack. These instruments give reliable long-term operation, have high resolution (of value in analysis of bagfilter emission dynamics), have no moving parts, and provide a representative measurement over the length of the rod (which is typically up to 1 m in length). Their major shortcomings are that calibration can drift over time, particularly if the average charge on the flowing particles changes, and that it is difficult to audit the technique with a surrogate (we know of no good surrogate for dust which accurately simulates dust interaction with the rod). The key application area for this technique has been in monitoring stacks associated with bagfilters, where the charge on the particles tends to remain relatively constant.
Forward-scatter monitors measure the intensity of a light scattered by particles in a forward direction from an incident laser beam or other light beam. These instruments provide a high-accuracy measurement, which can be audited with surrogate scattering bodies (for example, a glass slide with an opal surface). Their major shortcomings are that the measurement volume is relatively small and that it is difficult to keep optical surfaces clean (which can cause drift in the measurement) even using sophisticated air purges.
The present invention seeks to ameliorate at least some of the abovementioned problems.